1. Field of the Invention
This invention relates to an earth connection unit used for groundedly connecting a wire (mounted, for example, on a vehicle) to a vehicle body or the like in order to remove high-frequency noise carried on the wire.
2. Background of the Invention
For example, various sensors, etc., mounted in an automobile or the like are often supplied through an electric wire with noise transmitted from a noise source (such as a horn and a wiper motor) having a high frequency component, external noise from a neon sign and others, induction noise and other noises. There has been encountered a problem that such inputted noise causes a malfunction.
Therefore, in order to remove the above noise, it has been proposed to groundedly connect the wire (which carries the noise) to a vehicle body via an earth connection unit.
FIGS. 16 and 17 show the above conventional earth connection unit.
This conventional earth connection unit 1 comprises two electrical elements 2 and 3 each having a pair of lead terminals 2b and 2c, 3b and 3c extending from an element body 2a, 3a thereof, wire terminals 7 each including a wire connection portion 7a for electrical connection to a conductor 5a of a corresponding wire 5 and an external connection terminal portion 7c for electrically connecting the wire connection portion 7a to the exterior, relay terminals 9 made of a metal sheet and including a terminal connection portions 9a for electrical connection to the external connection terminal portion 7c of the corresponding wire terminal 7 and a lead connection portion 9b for being soldered to one lead terminal 2b, 3b of the corresponding electrical element 2, 3, an earth plate 11 made of a metal plate (or sheet) and including an earth connection portion 11a formed on and extending from one edge of a mounting plate portion 11b thereof so as to be fastened to an earth connection-purpose structural member (such as a vehicle body) and two element connection portions 11c formed on and extending from other edges of the mounting plate portion 11b so as to be electrically connected respectively to the other lead terminals 2c and 3c of the two electrical elements 2 and 3, a synthetic resin-made unit case 13 on which the above component parts are mounted, two opening covers 15 for being attached to the unit case 13, and two terminal covers 17.
The unit case 13 has a bottom wall 21, and a plate portion placing portion 21a for the placing of the mounting plate portion 11b of the earth plate 11 thereon, as well as two connection portion placing portions 21b for the placing of the respective element connection portions 11c thereon, is formed on an outer surface of the bottom wall 21. An element receiving portion for receiving the electrical elements 2 and 3 and two parts receiving portions each for receiving the corresponding relay terminal 9 and wire terminal 7 are provided at the inner side of the bottom wall 21.
A plurality of working openings 21c are formed through the bottom wall 21 of the unit case 13, and each relay terminal 9 and the one lead terminal 2b, 3b of the corresponding electrical element 2, 3 which are received in the corresponding parts receiving portion in contacted relation to each other are soldered together through the corresponding working opening 21c. 
When each opening cover 15 is inserted from the side surface side of the unit case 13 along the bottom wall 21 to be disposed over the working opening 21c, lock projections 15a formed respectively on opposite side edges of the opening cover 15 are engaged with respective engagement portions 21d formed on the bottom wall 21, so that the opening cover 15 is fixed to the bottom wall 21 to close the working opening 21c. 
Each wire terminal 7 mounted on the wire 5 is received and held in the terminal cover 17, and in this condition the terminal cover 17 is fittingly attached to the unit case 13 to cover an outer surface of the wire terminal 7 received in the unit case 13.
In the assembled condition of the above earth connection unit 1 as shown in FIG. 16, the conductor 5a of each wire 5 is electrically connected to the earth plate 11 via the electrical element 2, 3, and is grounded when the earth connection portion 11a of the earth plate 11 is fastened to the vehicle body. When capacitors of a predetermined capacity are used as the electrical elements 2 and 3, the above earth connection unit 1 is used as a noise filter for removing high-frequency noise carried on the wire 5 (see, for example, Patent Literature 1 and Patent Literature 2).
Incidentally, in the case of the above earth connection unit 1, the earth plate 11 is fixed to the plate portion placing portion 21a by engaging a plate clamp 24 (formed on and projecting from the plate portion placing portion 21a) in an engagement hole 11d formed through the mounting plate portion 11b. Further, an elastic engagement piece portion 11e formed on and extending from the side edge of the mounting plate portion 11b is engaged with an engagement projection 25 formed on and projecting from a side wall of the unit case 13, thereby locking the earth plate 11 to the unit case 13.
Patent Literature 1: JP-A-2006-109587
Patent Literature 2: JP-A-2007-311630
However, in the above earth connection unit 1, the elastic engagement piece portion 11e for locking the earth plate 11 to the unit case 13 is exposed at the outer side surface of the unit case 13, and therefore it has been feared that the elastic engagement piece portion 11e might be elastically deformed upon contact from the exterior, so that this locked condition might be canceled.
In addition, in the case where the wire 5 received in the earth connection unit 1 is arranged to pass near to the earth plate 11, it has been feared that the wire 5 might be damaged upon contact with an end edge 27 of the exposed elastic engagement piece portion 11e. 
Furthermore, in the case of the above earth connection unit 1, the two separate opening covers 15 are provided for respectively covering the working openings 21c, and therefore a corresponding number of cover inserting/attaching operations to the number of the electrical elements (2 and 3) to be received in the earth connection unit 1 need to be carried out, and this invited a problem that the number of the component parts as well as the number of assembling steps increased.